Add net feature for compiling to embedded targets.

[langyo]

I'm developing WASI modules that do not require TLS support. However, when compiling the code to WebAssembly (WASM), the build process fails due to dependencies introduced by the socket2 crate. This appears to be caused by implicit TLS-related features in the dependency chain. To address this, I propose implementing a feature flag that explicitly disables TLS functionality in the library. Such configuration would align with the minimal requirements for Redis protocol parsing while maintaining WASI compatibility.

Here's the demo:

/*
[dependencies]
tokio = { version = "^1", features = ["macros", "rt"]}
redis = { version = "^0.29", default-features = false, features = [
  "script",
  "aio"
] }
*/
use redis::{Cmd, ConnectionLike, RedisResult, Value, parse_redis_value};

#[derive(Clone)]
pub struct MockConnection;

impl ConnectionLike for MockConnection {
    fn check_connection(&mut self) -> bool {
        true
    }

    fn is_open(&self) -> bool {
        true
    }

    fn req_command(&mut self, cmd: &Cmd) -> RedisResult<Value> {
        let result = parse_redis_value("+OK\r\n".as_bytes())?;
        Ok(result)
    }

    fn req_packed_command(&mut self, cmd: &[u8]) -> RedisResult<Value> {
        let result = parse_redis_value("+OK\r\n".as_bytes())?;
        Ok(result)
    }

    fn req_packed_commands(
        &mut self,
        cmd: &[u8],
        _offset: usize,
        count: usize,
    ) -> RedisResult<Vec<Value>> {
        let result = parse_redis_value("+OK\r\n".as_bytes())?;
        Ok(vec![result; count])
    }

    fn get_db(&self) -> i64 {
        0
    }
}

async fn run_single<C: ConnectionLike>(mut con: C) -> RedisResult<()> {
    interval.tick().await;
    println!("> GET a");
    let result: String = Cmd::get("a").query(&mut con)?;
    println!("< {result:?}");
}

#[tokio::main(flavor = "current_thread")]
async fn main() -> RedisResult<()> {
    run_single(MockConnection {}).await?;
    Ok(())
}

To build and run this demo by WASI target, try:

cargo build --target wasm32-wasip1
# Install wasmtime first: https://github.com/bytecodealliance/wasmtime
wasmtime ./target/wasm32-wasip1/debug/demo.wasm

[nihohit]

I want to get this in, but this requires some more thought & design.
The first big issue with this change is that it breaks the library for users who don't use the default features. This is unacceptable.
On a lower level, the amount of #[cfg(net)] through the code is rather distressing - it reduces readability and accessibility of the code. We need to add better mod scopes, so that not every new module has to be attributed like this.
I recommend looking into #1514 - I think that the sans-io system will fit WASI well, and while it is a larger change, it doesn't break existing users, only opens more usage scenarios.